1. Field
The invention relates to a method of repairing a railroad rail having a defect in the top portion of the rail by creating a cutout and filling the cutout with an appropriate material, preferably a high carbon content weld material.
2. Description of Related Art
Railroads have to maintain their track to ensure safe operation of trains. Some of this maintenance is centered around the repair of rail defects. Railroad rails may be manufactured with internal defects or, as a result of wear-and-tear or fatigue, develop defects. These defects are found using non-destructive test methods. The Federal Railway Administration (FRA) mandates periodic ultrasonic testing of railroad rails to locate defects in the rail. When a defect is found, a temporary accommodation or a repair must be made to the track structure. Many of these defects are located in the top portion (i.e. the web or head) of the rail.
There are two common welding processes used to facilitate the repair of defects in railroad rails. They are the thermite welding process and the flash-butt welding process. Rails repaired using a flash-butt weld are typically stronger and higher in quality than those repaired using a thermite weld. Repairs made using the thermite process are initially less costly due to the labor and additional equipment cost components required using the flash-butt process. Additionally, rail defects may be temporarily repaired through the use of Joint Bar splices (mechanical joints). The rail integrity is best maintained by having the lowest number of joints (mechanical or welded) in the track.
When repairing a rail defect, a length of rail localized around the defect is removed from the existing rail. This creates a gap (typically 13 to 19 feet in length) in the rail. A rail plug is inserted in the resulting gap to make up for the bulk of the rail length removed. A weld is then made at each end of the rail plug, welding the rail plug to the existing rail, and creating a continuously welded rail.
Regardless of the welding process used to install the rail plug, there is a need to maintain the Adjusted Rail Temperature (ART). The ART is the temperature at which the rail contains no longitudinal thermally induced rail stresses. The track is not designed to allow the rails to contract and expand in response to environmental temperature changes. It is designed to constrain the rail and to allow the rail to have tension and compression. The amount of tension or compression is determined by the ART and the Current Rail Temperature (CRT). The ART must be controlled because too low of an ART can cause the rail to buckle when the CRT of the rail is too high and too high of an ART can cause the rail to pull apart when the CRT of the rail is too low. Buckles and pull aparts cause unsafe conditions and can cause serious accidents.
When a repair is accomplished by installing a rail plug, it is unlikely that the rail plug installed will be of the exact length necessary to maintain the ART of the rail. The ART of the rail is altered. As such, the installed segment will have a different ART than desired. The ART of the entire rail adjacent to the repair plug installation is changed. Management of the ART could be simplified if the rail was not severed during the repair of a defect.
A thermite weld can be used to weld the existing rail to a rail plug. A rail plug is cut to a length approximately two inches shorter than the length of the rail, containing the defect, which is being cut out. The rail ends to be welded are aligned. A sand mold is attached to both the existing rail and the rail plug around an approximate one-inch gap between the end of the existing rail and the end of the rail plug. The thermite charge is contained in a crucible immediately above the sand mold. After the mold is pre-heated, the thermite charge is ignited. The thermite charge creates molten steel which pours into the sand mold. As the molten steel solidifies, it forms a casting which bonds to, and is contiguous with, both the existing rail and the rail plug. In this manner, the rail plug is welded to the existing rail to form a continuous section.
The rail ends at the other end of the rail plug are aligned. A second thermite weld is made at an approximate one-inch gap at the opposite end of the rail plug, joining the rail plug to the existing rail. The area of the rail containing the thermite weld is not as strong as and is not of the same quality as a normal rail. Moreover, such welds are not clean as they can include numerous inclusions from the welding process. As such, the thermite welds typically require subsequent repairs in order to maintain the railroad rail in a safe condition. This method also requires the repair crew to transport a rail plug to the repair site and the section of the rail containing the defect away from the site.
A flash-butt weld can be used to weld the existing rail to the rail plug. A rail plug is cut to a length approximately three inches longer than the length of the rail, containing the defect, which is being cut out. Rail anchors are removed from the existing rail until the gap created by the removal of the defect containing the plug is three inches longer than the defect containing the rail plug. This can only occur when the CRT is below the ART. When the CRT is below the ART, the rail is in a longitudinally tensile condition. The rail plug is put in to place in the track. The rail ends to be welded are aligned. A flash-butt welderhead is clamped across the abutment of the rail plug and the existing rail. The flash-butt welding cycle is carried out. The welderhead passes a high current across the interface between the existing rail and the rail plug. The current produces arcing between the mating surfaces. The arcing produces heat in both rails as well as a “flashing” away of the surfaces. As the cycle progresses and sufficient heat has been generated, the welderhead forges the two pieces of rail together to form an essentially single rail. The flashing away of the rail and the forging of the rail consume about one and one half inches of the rail from the rail plug. In this manner, the rail plug is welded to the existing rail to form a continuous section. A shear die is then pushed across the weld to remove the upset material and to return the profile to the rail contour.
The rail ends at the other end of the rail plug are aligned. The flash-butt welderhead is moved to the other end of the rail plug and clamped across the abutment of the rail plug and existing rail. The rail consumed during the production of the first flash-butt weld of the rail plug has created a gap at the location for the second weld. The rails are stretched to close the gap and the flash-butt weld cycle is carried out. The flash-butt weld consumes about one and one half inches of the rail at the second weld location. The rail is now returned to the pre-existing tensile condition. Rail anchors are placed onto the existing rail. The flash-butt welding process is typically more costly than a thermite process but produces a cleaner and stronger weld. However, this method also requires the repair crew to transport a plug to the rail repair site and the section of the rail containing the defect away from the site.
When rail plugs are installed using either the thermite or the flash-butt welding process, the rail is taken out of service. This prevents the railroad from running revenue producing trains. Thermite and flash-butt welding trucks need to occupy the track. The installation of a rail plug and resulting two welds uses valuable track time and needs to be kept at a minimum.
Joint Bar splices are, essentially, a reinforcing clamp applied to the rail to effect a temporary repair. A Joint Bar splice is used when there is not enough time to perform a complete repair or when other repair materials are not available. A Joint Bar splice, by government regulation, is a temporary repair and must be replaced within about 90 days. The Joint Bar splice reduces the operational limit of the rail in the repair area.
In the area of gas shielded arc welding of railroad rails, several approaches have been taught, although they have not necessarily met with functional success in the field. These include U.S. Pat. Nos. 6,407,364, 6,278,074, 6,207,920 and 6,201,216, all entitled “Method and system for welding railroad rails” and U.S. Pat. No. 5,605,283 entitled “Weld joint between two rails arranged behind each other along a rail track.” Fixtures for rail welding are taught in U.S. Pat. No. 6,396,020 entitled “Rail welding apparatus incorporating rail restraining device, weld containment device and weld delivery unit.” A key portion of computer robotic control for rail welding is taught in publication WO 0195132 entitled “Gap Welding Process.” U.S. Pat. Nos. 5,605,283, 6,396,020 and WO 0195132 are all assigned to the same company as this application. All of the above patents, U.S. Pat. Nos. 6,407,364, 6,278,074, 6,207,920, 6,201,216, 5,605,283, 6,396,020 and WP 0195132 are incorporated by reference as if fully set forth herein.
In addition to the above, it has been found that the amount of heat introduced into the rail during welding or gap closure can produce a de-carburizing effect at the rail interface. This can, in turn, result in the migration of carbon from the rail as well as a change in microstructure and material properties.
Moreover, the welding process can introduce hydrogen (H2) into the final weld which has the effect of embrittling the weld material and causing a weld failure.
Steel used in high strength applications such as railroad track has a substantially uniform strength. When the ends of such material are welded through gas shielded arc welding such as that taught in U.S. Pat. Nos. 5,773,579, 5,877,868, 6,069,333, 6,166,347, 6,201,216, 6,207,920, 6,278,074 and 6,407,364. Using apparatus such as that taught in U.S. Pat. No. 6,396,020 and U.S. Application Publication No. 2002-170,884 or U.S. Pat. No. 5,605,283, strength variations across the weld fusion line are problematic.
Typical welding electrodes for joining material have a carbon content of 0.1% or less. While higher carbon content steel is known, forming that steel into welding electrode commercially is not accomplished.
Other prior art metal forming and treating techniques include drawing and annealing in a carburizing atmosphere although these procedures are not believed to have been used in combination in the production of welding electrode.
The metallurgical properties of welds generally have been discussed in a paper entitled “Effect of Carbon Content and Peritectic Reaction on Hot Cracking of Weld Metal of High Carbon Steel” authored by Koreaki Tamaki, Hiroshi Kawakami and Jippei Suzuki of the Department of Mechanical Engineering, Mie University, Kamihama-cho, Tsu, Mie, 514-8507, Japan. This paper provides general background.
Thus, it is desirable to provide a rail defect repair system that addresses above-identified issues and is acceptable to railroads for their use.